Conventional scanning laser ophthalmoscopes provide structural imaging and real-time eye tracking and targeted stimulus delivery. For examples, Sheehy et al., High-speed, image-based eye tracking with a scanning laser ophthalmoscope, Biomedical Optics Express, Sep. 19, 2012, describes a confocal scanning laser ophthalmoscope. This scanning laser ophthalmoscope includes a light source, a reflective mirror assembly, and an x- and y-coordinate scanner. In one optical design described in this publication, light exiting a super luminescent diode (SLD) is coupled into an acousto-optic modulator (AOM) before entering the system. The light is collimated and sent through a basic 4f series of lenses onto an adjustable aperture (A1). Light travels through three mirror-based telescope assemblies (f=250 mm) to the human eye. Light is then reflected off the retina and sent back through the system into the light detection arm. Another series of lenses in a 4f configuration relays the light to be collected by a photomultiplier tube (PMT). A 50 μm pinhole (1.95 Airy disc diameters for a 4 mm pupil) is placed at the retinal conjugate plane prior to the PMT for confocality. The intensity (I) of the signal is sent to a personal computer (PC) for readout. This system, like other similar systems in the conventional art, is however limited to a 5-degree field of view. Moreover, this system is not suited for certain applications requiring the tracking of large eye movements or rapid eye movements.
Furthermore, the scanning laser ophthalmoscope described in Sheehy et al., High-speed, image-based eye tracking with a scanning laser ophthalmoscope, Biomedical Optics Express, Sep. 19, 2012 tracks eye motion in the following manner. A reference frame is selected (usually the first frame to occur in a scanning laser ophthalmoscope movie). Each subsequent frame is broken up into a set number of strips that are parallel to the fast scanner. Each strip within a subsequent frame is then linearly cross-correlated with the reference frame to create a stabilized version of that subsequent frame. The (x,y) displacements required to stabilize this frame with respect to the reference frame are used to measure the relative cardinal motion of the eye. Every subsequent frame can then be redrawn to be aligned with the reference frame. This occurs in real-time so that the operator can see both the subject's actual retinal motion and the stabilized version of the retina side by side on the software interface. Using the real-time eye trace generated from the (x,y) displacements of each frame as described above, the timing of the stimulus delivery can be controlled to guide its placement to any targeted location on the retina.
One of the problems with choosing a single reference frame for eye tracking in this manner is that, when the eye moves perpendicular to the orientation of the strips that are used for eye tracking in the image, the new regions of the retina that are imaged may no longer overlap with the reference frame. This can lead to a non-uniformly sampled eye motion trace. Frame rates and image quality can also be limited when imaging or testing.
Furthermore, microperimeters are devices that are capable of simultaneous retinal imaging and subjective visual function testing. Microperimetry has clinical utility because it can provide functional correlates to the structure observed in retinal images obtained from patients with retinal disease. In some cases, the fidelity of this link between structure and function depends on two factors: (1) image resolution, which determines the structures that can and cannot be visualized in the retinal image; and (2) the precision and accuracy of visual stimulation. Because the eye is always in motion, many conventional microperimeters use image-based retinal tracking to deliver visual stimuli more accurately. As a result, (2) can be heavily dependent on (1), among other factors. Current microperimeters typically employ images captured over large fields-of-view (˜30 degrees) and with relatively low lateral resolution to track eye motion at the rate of image acquisition (20-30 Hz). This is explained in Midena, E. Perimetry and the fundus: an introduction to microperimetry. (SLACK Inc., 2007), which is hereby incorporated by reference for its description of microperimetry. In addition, Harmening, W. M., Tiruveedhula, P., Roorda, A. & Sincich, L. C. Measurement and correction of transverse chromatic offsets for multi-wavelength retinal microscopy in the living eye. Biomedical optics express 3, 2066-2077, doi:10.1364/BOE.3.002066 (2012) is hereby incorporated by reference for its description of chromatic aberration. Current systems do not offer much flexibility in terms of stimulus wavelength composition and are unable to measure and account for imprecision introduced by the chromatic aberration of the eye.
In addition, electroretinography (ERG) is a technique for measuring retinal function objectively. ERG involves placing an electrode on or near the front of the eye to detect the small electrical changes in the retina that are triggered by the presentation of visual stimuli on a computer monitor. Multifocal ERG (mfERG) is a variant of ERG that yields spatially-resolved measures of outer retinal function. Conventional mfERG devices have a spatial resolution much coarser than the scale of many disease-induced retinal abnormalities, such as retinal drusen, and are thus unable to fully characterize their functional implications. The spatial resolution of mfERG is primarily limited by low signal-to-noise ratios. One way to improve signal-to-noise ratios is to simply collect more data. However, protracted recording sessions with finer-grained stimuli have conventionally only yielded sensible data in cases where fixation was exceptionally stable, often in young and healthy subjects. This is due to the fact that eye movements during the recording session can shift the stimulus to different and unwanted parts of the retina from one moment to the next, with the resultant mfERG recording comprising activity measured from a broader swath of retina than originally intended. This is explained in Sutter, E. E. & Tran, D. The field topography of ERG components in man—I. The photopic luminance response. Vision Res 32, 433-446 (1992), and Poloschek, C. M. & Sutter, E. E. The fine structure of multifocal ERG topographies. J Vis 2, 577-587, doi:10.1167/2.8.5 (2002), which are both hereby incorporated by reference for their description of mfERG.